1. Field of the Invention
This invention relates to a polishing technique of a metal film, particularly to a polishing method adapted for the formation of multilevel metallization of an electronic circuit device such as semiconductor device.
2. Description of the Related Art
With a recent tendency to high integration and performance improvement of a semiconductor integrated circuit device (which will hereinafter be abbreviated as “LSI”), novel micro fabrication techniques have been developed. Chemical mechanical polishing (which will hereinafter be called “CMP (chemical mechanical polishing)”) is one of such techniques and it is frequently employed in an LSI fabrication process, particularly, planarization of an interlayer dielectric layer in a multilevel metallization process, formation of a metal plug and formation of inlaid interconnect. This technique is disclosed in, for example, U.S. Pat. No. 4,944,836.
Recently, there has been an attempt to use a low-resistant copper (Cu) alloy instead of the conventionally used aluminum (Al) alloy as a metallization material with a view to imparting an LSI with high-speed performance. The Cu alloy is however unsuited for micro fabrication by dry etching which has been frequently employed for the metallization using an Al alloy.
Accordingly, a so-called damascene method which comprises depositing a Cu-alloy thin film over an insulating film having a groove formed therein, and removing the Cu-alloy thin film other than a portion embedded in the groove by CMP, thereby forming an inlaid interconnect has been adopted mainly.
This technique is disclosed, for example, in Japanese Patent Application Laid-Open No. Hei 2-278822. It is the common practice to insert a barrier metal film such as titanium nitride (TiN) film, tantalum (Ta) film or tantalum nitride (TaN) film having a thickness of about tens of nm between the Cu-alloy thin film and the insulating film in order to improve their adhesion and form a barrier against Cu diffusion.
Conventionally, a polishing liquid used for CMP of a metal film such as Cu alloy upon metallization is usually composed mainly of a polishing abrasive and an oxidizing substance (which may be called “oxidizer”).
The principal mechanism of CMP is to, while oxidizing the surface of a metal film by the oxidizing action of an oxidizing substance, mechanically remove the resulting oxide by the polishing abrasive, which is disclosed in page 299 of “Science of CMP” ed. by Masahiro Kashiwagi, publ. by Science Forum K.K. on Aug. 20, 1997.
Although an alumina abrasive or silica abrasive having a particle size of several tens to hundreds nm is known as the polishing abrasive, the former one is usually put on the market as an abrasive for CMP of a metal.
As the oxidizing substance, hydrogen peroxide (H2O2), ferric nitrate (Fe(NO3)3) and potassium iodate (KIO3) are usually employed. They are, for example, disclosed in pages 299 to 300 of the above-described “Science of CMP”. Among them, hydrogen peroxide has come to be employed frequently in recent years, because it is free of a metal ion.
The formation of an inlaid interconnect or plug by using a conventional polishing liquid composed mainly of a polishing abrasive for CMP of a metal is however accompanied with the following problems (1) to (7):    (1) generation of dishing (a recess of a metallization member) or erosion (wear at the portion of an insulating film), (2) generation of scratches (by polishing), (3) occurrence of delamination, (4) necessity of abrasive removal by post-CMP cleaning, (5) a high cost of the polishing liquid, (6) a high cost relating to a polishing agent feeder and equipment for waste liquid disposal, and (7) dust in a clean room from a CMP apparatus.
The above-described problems result from CMP with an abrasive-containing polishing agent. In the conventional CMP method, however, a polishing abrasive is necessary for bringing about mechanical removing effects, thereby promptly removing the oxidized layer formed by an oxidizer and a practical polishing velocity is not available without the addition of a polishing abrasive.
The present applicants studied a method for fabricating an inlaid interconnect structure by polishing a metal film with an abrasive-free polishing liquid, which is disclosed in Japanese Patent Application Laid-Open No. Hei 11-135466. This method makes it possible to fabricate an inlaid metal interconnect structure by subjecting the surface of a metal film to mechanical friction with a polishing liquid containing an oxidizing substance, a substance which solubilizes the thus-oxidized substance in water and water and optionally an anticorrosive substance. For example, a Cu interconnect is formed by using an abrasive-free polishing liquid containing aqueous hydrogen peroxide, citric acid and benzotriazole (which will hereinafter be abbreviated as “BTA”).
Although the above-described problems (1) to (7) can be overcome by the use of the above-described abrasive-free polishing liquid, a removal rate of a metal such as Cu under the ordinary polishing conditions is 80 to 150 nm/min. Even if a down force as high as 300 g/cm2 or greater is applied, the removal rate is saturated and does not exceed 200 nm/min. Thus, there still remains a problem that a throughput cannot be improved further. A commercially available alumina-abrasive slurry attains a removal rate of 200 to 400 nm/min when a high down force is applied, but in this case, problems such as scratches or delamination becomes more serious.
Some official gazettes relating to the present invention have so far been published and an aqueous phosphoric acid solution is disclosed (in Example 4) in Japanese Patent Application Laid-Open No. Hei 7-94455 as one of abrasive-containing polishing liquids for Cu. According to it, a removal rate ratio of Cu relative to an insulating film can be increased even to 14.5 by using an abrasive-containing polishing liquid containing phosphoric acid in an amount of 3% (FIG. 5 of the above-described official gazette; when Cu is 100%).
Based on the experiment made by the present inventors, however, the removal rate exceeding 50 nm/min was not available only by combination of an abrasive and an aqueous phosphoric acid solution under the practical polishing conditions (under a down force of 500 g/cm2 or less at a platen rotational speed of 90 rpm or less). After the elimination of the abrasive, the removal rate was 20 nm/min or less. The results of the experiment suggest that the above-described abrasive-containing polishing liquid had a high removal rate ratio but could not attain highly-precise polishing (for example, free from erosion) in a sufficiently high throughput.
The polishing liquid disclosed by the present invention, on the other hand, has a removal rate as high as 500 nm/min or greater, which is superior by at least one figure to the above-described one.
Also in an abrasive-containing polishing liquid for CMP of tungsten, which is disclosed in Japanese Patent Application Laid-Open No. Hei 10-265766, phosphoric acid or an organic acid is employed as a stabilizer. In this case, the stabilizer serves as a chemical which suppresses the reaction of a catalyst (ferric nitrate) with an oxidizer (hydrogen peroxide) to be added to the polishing liquid.
According to the experiment by the present inventors, the above-described polishing liquid had a Cu etching rate of 100 nm/min or greater and could polish a Cu film, but an Cu interconnect disappeared by etching, suggesting that the polishing liquid was unsuited for CMP of Cu. The idea of the present invention to increase a removal rate of Cu by the addition of phosphoric acid to an abrasive-free polishing liquid does not come from this patent gazette.
In Japanese Patent Application Laid-Open No. Hei 11-21546, a polishing liquid for CMP of Cu is disclosed. The polishing liquid comprises a polishing abrasive, an oxidizer (ex. urea-aqueous hydrogen peroxide) and a complex-forming agent (ex. ammonium oxalate), a protection-layer forming agent (BTA) and a surfactant.
In the column [0025] or [0034] of the official gazette, described is the addition of an inorganic acid such as sulfuric acid, phosphoric acid or nitric acid in order to promote a removal rate of a barrier metal layer such as titanium or tantalum or to adjust the pH of the polishing liquid. The surfactant as described-in it serves to suppress sedimentation, coagulation or decomposition of the polishing abrasive. According to the experiment by the present inventors, it was substantially difficult to polish a Cu film with the above-described polishing liquid if it did not contain the polishing abrasive. In other words, the polishing liquid described in the above patent gazette removes a Cu oxide by making use of the mechanical removing action of the abrasive as essential action. From this patent gazette, the invention of adding a surfactant or thickener to an abrasive-free polishing liquid cannot be anticipated.
A polishing liquid not containing a free abrasive is disclosed in Japanese Patent Application Laid-Open No. Sho 52-21222 as a chemical polishing liquid for copper ornaments such as camera parts. The polishing liquid is made of a surfactant, hydrogen peroxide, sulfuric acid and phosphoric acid. The abrasive is incorporated in order to impart the copper surface with luster by polishing with an emery paper having an abrasive attached thereto. The surfactant is effective for imparting the polished surface with luster by improving the wetness on it. According to the experiment by the present inventors, however, this polishing liquid has an etching rate of 1000 nm/min or greater so that it is not suited as a polishing liquid for the fabrication of an inlaid Cu interconnect of several hundreds nm level.
The polishing liquid of the present invention is required to have a Cu etching rate of 10 nm/min or less because of the following reason. The thickness of the metallization layer of a semiconductor device to which the polishing liquid of the present invention is to be applied is usually 300 nm to 1000 nm. Considering that it takes several minutes for polishing, use of a polishing liquid having, for example, an etching rate of about 100 nm/min may etch Cu of the metallization portion even to the depth of several hundreds nm. In short, the depth of the dishing may reach several hundreds nm. In order to suppress the dishing depth to several tens rim or less, the etching rate by a polishing liquid must be decreased to 10 nm or less. The etching rate is preferably reduced to 1 nm/min or less in consideration of the over polishing time.
In Japanese Patent Application Laid-Open Nos. Sho 55-47382 and Hei 6-57455, an abrasive-free polishing solution is disclosed. In the former one, described is a chemical polishing liquid which is used for deflashing of an aluminum-made machined product and comprises an acid (including a phosphoric acid) and an aromatic Al chelate agent and optionally, a surfactant and hydrogen peroxide. In the latter one, described is a chemical polishing solution which is used for pretreatment of bronze plating and comprises hydrogen peroxide, oxyquinoline, a complex-forming agent and a surfactant, and optionally a phosphoric acid or sulfuric acid which serves to adjust luster or satin. The surfactant is added to improve wetness and prevent generation of mist due to foaming. Either one has an etching rate of 100 nm/min or greater and polishes by its etching action (without friction). Accordingly, it is not suited as a polishing liquid for the fabrication of an inlaid Cu interconnect of the present invention. In addition, the polishing liquid for the inlaid interconnect of LSI which is the object of the present invention is required to attain flatness of a nanometer level, higher than the level of flatness (luster) attained by the polishing liquids described in the above official gazettes.